Interference Mitigation In Radio Astronomy and Remote...

Interference Mitigation In Radio Astronomy and

Remote Sensing

Feb 10, 2004

Steve Ellingsonellingson@vt.edu

The First Radio Astronomer

…And discovers the Galactic Center!

1932: Karl Jansky given task to find sources of radio frequency interference (RFI) to transatlantic radio communications

First Sky Maps at Radio Frequencies1938-48: Radio engineer Grote Reber builds the first modern radio

telescope (in his backyard!) and makes the first sky maps

First Sky Maps at Radio FrequenciesAnd may also have been the first to grapple with RFI…

Oh no…RFI !!!

1967: While studying interstellar scintillation, Antony Hewish and Jocelyn Bell puzzle over mysterious interference

…And realize that they have discovered a pulsar!

Sometimes interference is…something new

1967: While studying interstellar scintillation, Antony Hewish and Jocelyn Bell puzzle over mysterious interference

…And realize that they have discovered a pulsar!

Sometimes interference is…something new

But, most RFI is just that – interference.

A Present Day RFI Problem: GLONASS vs. OH

Hydroxyl (OH)– Important for the study of evolved stars & Galactic dynamics– Spectral lines in at least three bands between 1600-1800 MHz– Bands protected world-wide by ITU regulation

GLONASS– The Russian Version of the US Global Position System (GPS)– 24 carrier frequencies over 1602-1616 MHz

Narrowband (500 kHz) DSSS emission

Wideband (5 MHz) DSSS emission

Typical GLONASS Power Spectrum

By the way:INR < −27 dB

in occupied bandwidth

What GLONASS does to Aperture Synthesis Imaging

What GLONASS looks like to a Radio Astronomer

Up to 50% of data collectedin this band has to be

thrown out!

Why Not Just Throw Away Bad Data?

A typical major radio telescope costs on the order of US$20,000 per day to operate

Post-observation editing & scheduling is possible, but is – labor intensive– subject to selection effects

There are only a few major radio telescopes, so telescope time has value that transcends simple dollar estimates

In summary: Real-time in-line RFI mitigation is highly desirable

Adaptive Canceling Approach

Don’t need to knowanything about interferencewaveform

Requires large INR

Injects referencenoise into output

Courtesy R. Fisher (NRAO)

Adaptive Canceling Approach

Don’t need to knowanything about interferencewaveform

Requires large INR

Injects referencenoise into output

May be possible to patch this up…current research topic

Otherwise, best hope is to exploit waveform knowledge

Courtesy R. Fisher (NRAO)

Ellingson, Bunton, & Bell (2001), ApJS, 135, 87

GLONASS Canceller

GLONASSDemodulator

Noise-Freecopy of GLONASS

as transmitted

The Hard Part:Modeling the effect

of antennas &receiver passbands

Noise-Freecopy of GLONASS

as received

Ellingson, Bunton, & Bell (2001), ApJS, 135, 87

GLONASS Canceller

Estimate System Response

By Correlation

GLONASS Canceller

Ellingson, Bunton, & Bell (2001), ApJS, 135, 87

Australia Telescope Compact Array (ATCA)Narrabri, NSW

Observations of OH MaserIRAS 1731-33

(4 sec integration)

About 24 dB suppression,

Limited by weak INR

Another Problem: L-Band Radar• The band 1215-1400 MHz is important for:

• Spectroscopy of redshifted HI• Continuum & Pulsar work• Earth climate & geophysical studies: Brightness temperatures infer ocean

salinity, soil moisture, ...

• AND this entire band is allocated primarily to aviation radars!

Waveform: Fixed-frequency (CW) or chirped,& pulsed

Pulse length: 2-400 µsPulse spacing: 1-27 ms (typical duty cycle ~0.1%)Bandwidth: ~1 MHzTx pwr: 103 - 106 WAntenna: Highly directional,

rotation rate ~10 s

Another Problem: L-Band Radar• The band 1215-1400 MHz is important for:

• Spectroscopy of redshifted HI• Continuum & Pulsar work• Earth climate & geophysical studies: Brightness temperatures infer ocean

salinity, soil moisture, ...

• AND this entire band is allocated primarily to aviation radars!

Waveform: Fixed-frequency (CW) or chirped,& pulsed

Pulse length: 2-400 msPulse spacing: 1-27 ms (typical duty cycle ~0.1%)Bandwidth: ~1 MHzTx pwr: 103 - 106 WAntenna: Highly directional,

rotation rate ~10 s

I’m focusing on radio astronomy,but virtually all comments areequally applicable to all forms

of microwave radiometry,including remote sensing.

L-Band Interference Surveyor/Analyzer (LISA)LISA co-observes with existing

passive microwave sensors to identify sources of damaging RFI

• Spectrum analyzer for full-bandwidth monitoring of power spectral density

• 14 MHz (8+8 bit @ 20 MSPS) coherent sampling capability for waveform capture and analysis

Nadir-lookingcavity-backed spiral

antenna w/ custom LNA & calibration electronics

in tail radome

Spectrum analyzer,electronics rack &control console

mounted in cabin

RF distribution, antenna unit control &

coherent sampling subsystem

20,000 ft over Virginia/Maryland Coast

OSU NASA/IIP Wideband Digital Receiver

200 MSPSA/Ds

FPGAs implementing Receiver, FFT, RFI Mitigation, PC Interface

What Arecibo Sees:

Measured Radar Waveform Characteristics

Derived Transmit Pulse Waveform(Based on receive data taken at Arecibo

and lots of post processing)

Magnitude

Phase

PSD

Ellingson & Hampson (2003), ApJS, 147, 167.

Derived Channel Impulse Response

Pico de Este to Arecibo

Max hold

Mean

Ellingson & Hampson (2003), ApJS, 147, 167.

Every detected pulse infers the presence of many delayed

(possibly undetected) pulses

Asynchronous Pulse Blanking (APB)Continuously estimates mean/variance of incoming time domain signal

A sample > β standard deviations above the mean triggers blanker

Blanking operates on down-stream data exiting a FIFO; blanking window extends before and after triggering sample

APB blanking decision

Triggering pulseMultipath copies

APB in Action @ Arecibo

Total power in 50 MHz x 42-ms integrations

APB offAPB on

APB in Action @ Arecibo

Total power in 50 MHz x 42-ms integrations

APB offAPB on

Works great for continuum & spectroscopy,also total power remote sensing

(But still INR-limited)

Blanking is Not For Everyone…Blanking is problematic for “time-domain” radio astronomy

– Pulsars– Asynchronous or one-time astronomical transients

“Giant” Pulses from pulsarsPrompt emission from GRBsPrompt emission from SNeCoalescing neutron star & black hole binariesExploding black holes

In dealing with these things, we would really like to be able just to “look through” the interference; i.e., back to canceling

Possible answer is Pulse Canceling: Estimate and subtract pulses, as opposed to simply blanking

Pulse Canceling at Arecibo

Before

After (~16 dB suppression)

Ellingson & Hampson (2003), ApJS, 147, 167.

Pulse Canceling vs. Pulse Blanking

Before

Pulse Canceling

Pulse Blanking

Ironically, it is thedetector - not the

waveform estimation -that limits performance.

Ellingson & Hampson (2003), ApJS, 147, 167.

A Brief Comment on Pulse Searches…

A Brief Comment on Pulse Searches…

Will an RFI pulse canceller eat astronomical pulses too?

Dispersive Interstellar Medium

A Brief Comment on Pulsar Observing…

Asymptotic vs. Known Dispersion Measures

Cordes & Lazio (2003)

So Could Interesting Pulses Be Thrown Out?

DM < 20 or so: Risk decreases withdecreasing DM

DM=15

DM=5DM=0.5

DM > 20 or so: No, if time abovethreshold is taken into account

DM=71

DM=25

DM=∞

Radar

DM ~ 20: Dispersed pulse looks like radar at output of matched filter.At Arecibo, dispersed pulses greater than about 0.1-1.0 Jy are detected.

In general: Risk is greatest when dispersed pulse exhibits the sametime-frequency occupancy as radar pulse. (In this case, 7 ms x 150 kHz)

Ellingson & Hampson (2003), ApJS, 147, 167.

Other Annoyances…

FPS-117 radar received at Arecibo(Linear FM Waveform)

Typical Pulsar Pulse

Ellingson & Hampson (2003), ApJS, 147, 167.

“RFI-Challenged” ScienceAsynchronous & one time pulses (already discussed)

Deuterium – Very long integration at 327 MHz– Detection, Measurement

The 21-cm Signature of the Epoch of Reionization (EoR)– Very long integration in 75-225 MHz band– Detection, Mapping– Critical (justifying) science for LOFAR

21-cm emission from galaxies at very-high redshift– 1420 MHz ends up in the 1215- 1400 MHz Air Traffic Control Radar band– Critical (justifying) science for SKA

“RFI-Challenged” ScienceAsynchronous & one time pulses (already discussed)

Deuterium – Very long integration at 327 MHz– Detection, Measurement

The 21-cm Signature of the Epoch of Reionization (EoR)– Very long integration in 75-225 MHz band– Detection, Mapping– Critical (justifying) science for LOFAR

21-cm emission from galaxies at very-high redshift– 1420 MHz ends up in the 1215- 1400 MHz Air Traffic Control Radar

band– Critical (justifying) science for SKA

These experiments are unlikely to succeed unless effective RFI mitigation

technology is worked out!